The present invention relates to a system for reducing distortion present in an alternating current power system. More particularly, the present invention is a system and a method for reducing distortion in the alternating current power system using an active filter and a passive filter.
Alternating current (AC) distribution power systems operate most efficiently when a distortion free sine wave voltage is present. When an AC distribution power system has non-linear loads, typically distortion voltage and current are introduced. Attempts to correct the distortion voltage and current have included the use of both passive filters and active filters used by themselves or in combination. In passive filtration, a filter of fixed impedance is connected to the power system at or near the point of common coupling with a load. The passive filter provides a low impedance path for the distortion currents, thus bypassing them from entering the power system.
Significant drawbacks are present with this filtering technique. For example, since the passive filter is in parallel with the power system impedance, a resonance condition may result and could cause an over-voltage condition at the point of common coupling. In addition, besides "shunting" or "sinking" distortion currents generated at the point of common coupling, the passive filter further sinks currents generated elsewhere in the power system. Consequently, the power rating of the passive filter components must be increased to handle the additional current requirements.
Active filters have also been connected to the power system to reduce distortion on the power system. When active filters are used, distortion currents present at the point of common coupling are measured on the load side of the power system. Typically, the active filter includes a power electronic converter that either supplies or draws the distortion currents present at the point of common coupling so they are not present on the power system. However, since fundamental frequency voltage is present across the active filter, and since the distortion current can be significant in comparison to the fundamental frequency current, the power rating of the active filter must be high thus making this filtering technique quite expensive to implement.
Techniques have also been advanced wherein the passive filter and the active filter are connected in series, the combination of which being connected to the power system across the load. The passive filter passes the distortion current of the active filter to the power system and, at the same time, presents a relatively high impedance to the fundamental component of current such that the fundamental voltage is substantially across the passive element and not across the active filter.
Although in theory, use of the series connected passive and active filter arrangement provides the benefits discussed above, implementation is yet a different matter particularly when used on utility power systems where thousands of volts are typically present. If not connected properly, voltages and currents under transient conditions and startup could easily damage filter components.